Hey Yves!
In WooCommerce you can only use the eCurring products that have the mandate type set to ‘Online Payment (First term)’ (Dutch: ‘Online betaling (Eerste termijn).
Could it be that the eCurring products you’ve added have a different mandate type? If so, you need to change the mandate type of the eCurring product to ‘Online Payment (First term)’ (https://prnt.sc/nxj426).
ps. what is the name of your eCurring account?
All the best,
Julian